Many commercial industries employ various mixing and extrusion mechanisms such as, for example, a screw extruder, in a variety of processing operations. Such screw extruders are employed in plastics applications such as, inter alia, polymer compounding, production, and finishing; incorporation of additives such as flame retardants, cross-linking agents, and plasticizers; production of polymer blends and alloys; development of multi-purpose of concentrates; incorporation of fillers and reinforcement materials' such as glass, talc, clay, carbon black, etc.; extraction of volatiles such as moisture, solvents, monomers, and oligimers; dispersion in size reduction of pigments; polymerization, pplycondensation, polyaddition, and grafting reactions. Screw extruders, including particularly twin screw extruders, are also employed in the continuous processing of rubber compounds and the production of powder coatings for appliances, automotive, architectural designs, lawn and garden items, general metal finishing items, and functional protective coatings as well as for direct extrusion of sheets and films, cross-linkable and foamable films, profiles, tubing, fiber spinning, wood fiber-plastic composites, and semi-permeable membranes. In other instances, screw extruders have been used in the production of commercial chemicals such adhesives and sealants, agricultural products, ceramics, elastomer production, starch modification, carbon extrusion, pharmaceuticals, petroleum additive processing, and energetics, as well as in the food industry for the production of snack pellets, chewing gum, final mixes, candy cooking and caramelizing, protein texturizing and pet food and for processes that include extrusion cooking and starch gelatinazation.
Screw extruder throughput, i.e., the rate at which a screw extruder can extrude one or more materials to make a particular product, directly affects the profitability of an overall process as well as the efficiency with which a market demand can be met. There is, therefore, always a need to improve the throughput of a screw extruder.
Additionally, as is well known in the industry, build-up of degraded material within a screw extruder, and again particularly a twin screw extruder, can produce adverse effects on the quality of the extruded product. Degraded material typically accumulates at dead zones within the extruder where the rate of fluid flow is either greatly diminished or static. Dead zones are the result of an interruption of the fluid flow within the extruder barrel that allows a portion of the subject material to degrade as a result of extended exposure to elevated temperatures and/or mechanical energy. Degraded material can form a solid coating on both the screw elements and barrel sections, and physical properties of a degraded material are often inferior to a virgin product. Portions of the degraded material often end up in the finished product and thereby reduce the overall product quality.
Efforts to remove degraded material, from screw extruders may typically involve running purge compounds through the extruder. However, there are instances where purging the extruders will not suitably cleanse the extruder. In these instances, the extruder must be at least partially dismantled and cleaned by hand. That is, cleaning out degraded material involves removing the screw, i.e., the conveying and kneading elements, from the extruder barrel and hand-cleaning both the screw(s) and barrel. Oftentimes, the extruders will have multiple shafts and each and every screw will need to be cleaned. Both of these methods, as well as any other cleansing methods, require that production be stopped, thereby resulting in lost of production and in expense incurred in failing to produce during down time.
There is, therefore, a need to eliminate dead zones with a screw extruder. In doing so, the relative rate at which degraded material forms within a screw extruder will be advantageously reduced. In addition, a need continues to exist, for a screw extruder assembly that can be cleaned easily and effectively so as to lower cleaning costs.
A number of differently configured conveying elements have been manufactured for use in modern screw extruders. Conveying elements are those elements of a screw whose essential purpose is to convey material through the extruder. To do so, they typically have extremely small clearances between them and the barrel or bore such that only a de minimus amount of material may be allowed to flow over the ends of the flights of the conveying element.
Typical conveying elements generally comprise one or more flights forming a generally spiral structure. For many conveying elements, two sets of opposing flights are employed and spirally wound around a central portion suitable for receiving the shaft of the extruder. By “opposed,” it is meant that upon examining a cross-section of the element with these two flights, the flights will be opposite each other with the shaft receiving bore therebetween. Moreover, for conventional conveying elements, the slope of the flights will rise and fall consistently and, therefore, the ends of the elements will have a consistent elipscoidal or football-shape cross-section to them.
It is also known to use conventional conveying elements which have only one flight or as many as three flights on them. While the conveying elements having only one or three flights on them will not have opposing flights, or elipscoidal cross-sections, the slope of the flights will rise and fall in a generally consistent and uniform pattern which is well understood by those skilled in the art such that the peripheral profile of the conveying element will be convex in nature. For purposes of describing the present invention as compared to the prior art, the terms “conventional conveying elements” and/or “standard conveying elements” shall mean these types of elements.
However, not all conveying elements are conventional. Many extruders now employ “modified” conveying elements that are specially designed to improve the performance of the extrusions taking place. By the term “modified,” it is meant that the conveying elements (and, later the kneading elements) do not have conventional, cross-sectional profiles of a completely convex nature. Generally, these modified. conveying elements may have differently angled or sloped flights with respect to each flight's front and back. That is, while the back slope of a flight may be essentially the same slope as for a standard conveying element, the front slope may be much steeper, or vice versa. Such a configuration of flights of the conveying element provides a completely different shape to the cross-sectional configuration of the conveying element. In the instance of a two opposed flight-modified conveying, element, the, element will have an “S-shaped” cross-section rather than a “football-shaped” cross-section; It will be appreciated that if both slopes are significantly steeper, then the cross-section configuration will be “narrower” at the tips of the flights. In this situation, the conveying element will have at least a part of the cross-sectional profile be concave in nature.
Such modified conveying elements having S-shaped cross-sectional profiles, hereinafter sometimes referred to as “S conveying elements,” fare well-known in the art. However, while S conveying elements and other modified conveying elements have gained in popularity within the extrusion industry, the industry has continued to use conventional kneading elements. Those modified conveying elements having even narrower flights, i.e., are highly sloped on both the front and back of the flight, will sometimes be referred to hereinafter as “SS conveying elements.”It will be appreciated that such S conveying elements and SS conveying, elements are typically differentiated and modified from those conventional. conveying elements having two “normal” opposing flights. However for the present invention, it will be understood that any modified conveying element modified to include a flight that provides a cross-sectional profile with at least a portion of the periphery thereof being concave in nature will be included as a modified conveying element, regardless of the number of actual flights.
In preparing a screw extruder for use, both conveying and kneading elements are typically used. Kneading elements are used not only to melt, mix and knead the extrusion mixture, as its name implies, but also to create a gap between conveying elements so that, where there are changes in the configuration, size or shape of the conveying elements or where the processing temperature or pressure might change, the kneading elements allow for these changes to occur over a greater period of time or along a length of the screw, not instantaneously, which could be deleterious to the extrusion mixture. Kneading elements are differentiated from conveying elements in that their essential purpose is to melt, mix and knead the materials that pass through the kneading zone of, the extruder. To do so, kneading, elements are designed to enhance flow between their periphery ridge (i.e., their outer edge) and the barrel or bore of the screw extruder as well, as between the kneading blocks themselves, as compared to conveying elements used for the same barrel or bore. This enhancement allows material to flow over the end of and between the kneading element which, in turn, allows for greater dispersion and/or distributive mixing of the material as compared, to that done in the conveying zone by the conveying elements.
Heretofore, conventional conveying elements and conventional kneading elements (conventional kneading elements being hereinafter defined as having the same cross sectional profile as the conventional conveying elements) were aligned in a contiguous manner such that the material being extruded would flow from the conveying element to the kneading element without being interrupted in any manner. However with the advent of modified conveying elements like those described above, it should be appreciated that, while the conventional kneading elements are commonly aligned and contiguous with the modified conveying elements, the interface between a modified conveying element that is aligned and contiguous with a conventional kneading element unfavorably creates an interruption in the material flow within a screw extruder. The interrupted material flow occurs because the cross-sectional profile of the conventional kneading element and the cross-sectional profile of the modified conveying element are peripherally incongruent. For example, where an S conveying element is used with a conventional kneading element, the S-shaped profile of the S conveying element does not match the football-shaped profile of the conventional kneading element. There is, therefore, a need to eliminate this cross-sectional peripheral incongruency between a modified conveying element and a kneading element.